both prompt() and document.write() are considered obsolete - the first is only really for debugging purposes these days and the second has one very obscure use that has nothing to do with what you are doing.

Form elements are the preferred method for getting user input and either DOM methods or innerHTML are better ways of displaying dynamic content.

At the very least I would put line breaks into my output.

From what I can see, there is nothing really to loop over or put into an array. Sometimes code is cumbersome. If you were to continue with document.write you could get all the variables at the start and use one call to display everything...

Code:

<script>
var userLength = prompt("What is the length (m) of the area");
var userWidth = prompt("What is the width (m) of the area");